In order for it to be able to interrupt a conductive connection, a circuit breaker has at least one switching contact. The conductive connection is intended to be interrupted when the current level of a current through a power line exceeds a particular value. This requires the current level to be measured. In the present case, a Rogowski converter is used to measure the current level; this is a coil without an iron core, i.e. an air-core inductor or generally a coil wound on a plastic body. The Rogowski converter has an amplification stage arranged downstream of it for amplifying output signals from the Rogowski converter, that is to say for amplifying the voltage induced in the Rogowski converter on account of a change in the current level through the power line. There is also a data processing device—typically a microcontroller—for evaluating the thus amplified output signals and for prompting at least one of the switching contacts to open on the basis of the result of the evaluation.
A special feature of the circuit breaker of the type in question is that it has a device for applying a DC voltage to the Rogowski converter. The purpose of this is to check the Rogowski converter for wire breakage: if a wire in the coil is damaged, a voltage offset is obtained at the output, said voltage offset being detected by the data processing device. This allows the data processing device to establish whether or not the Rogowski converter is functional.
It is now desirable to use one and the same type of electronic trip unit (ETU) for a multiplicity of switch rated currents. Depending on the rated trip current level, the amplifier stage of the ETU needs to amplify the output signals differently.
There is now the problem that when the Rogowski converter has a DC voltage applied to it, the offset is amplified differently depending on the gain factor in the amplifier stage. Since the data processing device has only a restricted input range for the measured values, this results in restrictions in the use of the ETU of the circuit breaker of the same type for different switch rated currents—the measurement range of the data processing device is not sufficiently large.